The present invention relates to a process for the polymerization of vinyl chloride in aqueous suspension and the use of the polyvinyl chloride powders obtained by this process in the production of separator plates for batteries and accumulators (battery separators).
The qualities demanded of the powdered resins to be used in the production of battery separators include good wettability and a low average particle size that will ensure that separators with low diameter pores are obtained. It is for this reason that polyvinyl chloride powders obtained by polymerization in aqueous emulsion are customarily recommended for use in this particular application. It has in fact been found that powdered polymers obtained in aqueous emulsion exhibit good processing characteristics on the whole. Thus, they can be sintered and can be readily applied with a doctor knife or roll. Furthermore, the wettability of the resulting separators is generally satisfactory. Nevertheless, they are unsatisfactory as regards resistivity and mechanical strength and tend to cause foaming due to their high emulsifier content.
Published French Patent Application No. 71.20132 filed on June 3rd, 1971, in the name of the Sumitomo Chemical Co. Ltd., (and corresponding U.S. Pat. No. 3,766,106) describes the production of polyvinyl chloride powders by the polymerization of vinyl chloride in aqueous suspension in the presence of an oil-soluble initiator and a dispersing agent system comprising a cellulose derivative and an anionic emulsifying agent. It is stated therein that a powdered polyvinyl chloride of this kind exhibits good mechanical and electrical properties, but is nevertheless unsuitable for the production of battery separators when used alone because of its poor application characteristics, whether a doctor knife or roll is used, and particularly because of its poor sintering properties.
The same patent recommends using, in the production of battery separators, compositions comprising a mixture of 95 to 50% by weight of a powdered polyvinyl chloride of this kind prepared by polymerization in aqueous suspension and 5 to 50% by weight of a powdered polyvinyl chloride prepared by polymerization in aqueous emulsion together with 0.1 to 3 parts by weight per 100 parts of the mixture of a non-ionic antistatic agent. The addition of polyvinyl chloride powder obtained in aqueous emulsion and a non-ionic antistatic agent in the proportions specified above admittedly improves the sintering characteristics and the machining properties of the polyvinyl chloride powder obtained in aqueous suspension. However, the mechanical properties of the resulting sintered separators are impaired and the separators always exhibit a tendency to cause foaming.
It would therefore appear that the battery separators obtained from the prior art compositions do not possess optimum properties. Furthermore, the preparation of such compositions is complicated due to the fact that it requires the production of polyvinyl chloride powders from two different formulations and their eventual mixing with an antistatic agent.